Abstract: A methodology has been developed to document the tremendous
growth large metropolitan regions have experienced over time. A geographic
information system (GIS) was used to compile a database of urbanization for the
San Francisco/ Sacramento urban region spanning 140 years. Historical records,
USGS topographic maps, aerial photographs and Landsat imagery were used to
identify the urban spatial extent. Digital transportation data and tabular
census data were also incorporated into the database to provide a more complete
picture of changes occurring over time. A time-series animation of urban
growth for the urbanized region depicts the alarming growth patterns the area
experienced between the mid-1800s and the 1990s. The same process is being
used to document growth in other urban regions, such as the
Baltimore-Washington area. This innovative use of
temporal spatial data and
animation focuses attention on the dramatic increases in urban development and
the spatial patterns that have developed over time.

INTRODUCTION

The United States Geological Survey (USGS) has a long tradition of mapping
land use and land cover, both current and potential. Initially charged by
Congress with the "classification of the public lands", the USGS began
topographic and geologic mapping in 1879. Early mapping activities took place
in the largely uninhabited Western United States. In the 1970s, using
high-altitude aircraft data, the USGS developed a land use and land cover
classification system for the nation (Anderson, et.al., 1976). A program was
implemented where a series of land use and land cover maps were produced by
aerial photo interpretation. Total coverage of the coterminous United States
was completed by 1986.

The USGS also developed techniques for mapping land cover using satellite
data. A project is now underway to produce a data base for North America
consisting of three Landsat Multispectral Scanner scenes representing land
cover in the 1970s, 1980s and 1990s. USGS also produces bi-weekly composites of
Advanced Very High Resolution Radiometer (AVHRR) data for the conterminous US
from which a prototype land cover characteristics data set was derived
(Loveland et. al., 1991). Current plans are to produce a Multi-Resolution Land
Characteristics Monitoring System in collaboration with several federal
agencies.

In 1993, the USGS began a Human-Induced Land Transformations (HILT) project
as a contribution to the US Global Change Research Program. The objective of
this project is to understand urban growth and its associated patterns from a
historical and a multi-scale perspective. Temporal mapping of major urban
regions will enable others to assess ecological, environmental and climatic
impacts of urban change and to model and predict future urbanization patterns
and impacts. To understand the urbanization process, we first focused our
efforts on a rapidly growing region in Northern California. Visualizing the
results of this project on a regional scale depicts the alarming rate at which
urban areas are growing together. We have also expanded our efforts to map the
growth of other large urban areas, including the Washington-Baltimore area.

Study Area

The urban area selected for this study includes the San Francisco Bay Area and
a portion of the Central Valley centered on Sacramento. (Figure 1) This region
encompasses a diverse environment from the Pacific Ocean to the Sierra Nevada
foothills and includes two large metropolitan areas (San Francisco and
Sacramento), and several rapidly growing cities (Stockton, Modesto, Tracy,
etc.) These urban areas were chosen because they exemplify the fast pace of
growth occurring both within and between urban areas that were once thought to
be quite separate.

Urban boundary information was derived from a variety of sources ranging in
scale and resolution.(Table 1) Data sources included historical accounts,
topographic maps, aerial photography and Landsat Multispectral Scanner and
Thematic Mapper data. Based on the available materials, seven data layers
between 1850 and 1990 were compiled for the San Francisco-Sacramento urban
regions.

Urban Definitions

Delineating an urban boundary first requires selecting a definition of urban
land. Urban land can be broadly divided into functional and physical
definitions. "Urban" in functional terms relates to activities such as
industrial, residential, agricultural, etc. However, there are often problems
determining which activities should be adopted as urban. Similarly, "urban" can
be defined in physical terms, relating either to population density or to land
cover, where any developed land is considered urban regardless of its function.
There are also variations in the intensity of land uses that influence the
definition of urban area, such as high or low housing density. For example,
the US Bureau of the Census primarily bases its definition on population size
and density:

An urbanized area comprises a place and the adjacent densely settled
surrounding territory that together have a minimum population of 50,000 people.
The "densely settled surrounding territory" adjacent to the place consists of
the following:

1. Territory made up of one or more contiguous blocks having a population
density of at least 1000 people per square mile...(US Census Bureau, 1990)

Depending on the specific purpose and the sources used for a study, there can
be more than one definition of urban land. We utilized a variety of sources in
this study, therefore several urban definitions were required. These sources
included satellite imagery, topographic maps and regional land use maps.

Landsat TM and MSS. Using satellite imagery, the spectral reflectance
value of an urban surface was used to identify urban land cover (Colwell,
1983). In color infrared images, urban surfaces, such as concrete and asphalt,
are characteristically bluish to white in color. Vegetation appears light to
dark red, and soil appears yellow to light brown. Individual buildings can be
identified if they are large and contrast with the background reflectance
color. The generalized urban boundary may contain undeveloped land that is
completely surrounded by developed areas. Parks, golf courses and other area
whose natural vegetation has been significantly altered is included in the
urban category.

Topographic Maps. Modern USGS Topographic maps typically use a pink
tint to indicate urban areas and a purple tint to indicate updated
photo-revised urban areas. These areas are defined by building and road
network density. However, earlier maps constructed around the 1900s did not
contain tinting , just the existing road network. Early cities were typically
constructed in a gridded road network pattern. In these cases, the urban
boundary was defined by these gridded areas, or the density of the road
network. (Figure 2) The pink and purple tinted areas were used as urban
boundaries for their respective dates. We often found that the tinted areas
did not include some commercial/industrial buildings (indicated as black
squares on the maps) and dense road networks that existed on the urban fringe.
In those cases we included those areas as part of the urban development.

ABAG Land Use Maps. The Land Use maps compiled by the Association of
Bay Area Governments were also used to derive urban delineations. These maps,
originally derived from aerial photo interpretation, consisted of five
residential classes, one commercial class, one institutional class, three
industrial classes, six open area classes, one military class, one
transportation class and one water class. Our interest was only to distinguish
urban from non-urban areas, so we defined urban as all residential classes,
except the very low density class, industrial, commercial, institutional,
military, transportation, and cemeteries (open) classes. We excluded the very
low density (<= 1 families per net residential acre) residential class
because those areas were not discernible as urban on the TM satellite
imagery.

Database Development

1990 and 1974. Two adjacent Landsat Thematic Mapper (TM) scenes, dated
June 6, 1990, and two Multispectral Scanner scenes dated July 16, 1974 were
used in this study. The spatial resolution of TM imagery is 28.5 meters and of
MSS imagery is 57 x 79 meters. The TM imagery was used to define which cities
would be included in the database and to delineate urban areas for 1990. A
150-square meter area (5x5 TM pixels or 3x3 MSS pixels) was considered the
minimum mapping unit for delineation. Any area that was visible on the TM
scene was to be included in the data base and delineated on the historic maps.
If an area was not visible, i.e. it was too small or was located in a dense
tree cover area, to maintain consistency, those areas were excluded from the
study. The imagery was first registered to a UTM coordinate system, then used
as a backdrop in the GIS to digitize polygons representing urban areas
on-screen. Aerial photography, local maps and local knowledge of the region
were also used to help interpret questionable areas on the imagery. Urban
areas were more difficult to identify on the MSS imagery because of its coarser
resolution. The 1990 data layer was used to help visually interpret the 1974
MSS imagery.

1954 and 1962. The ABAG maps were used to identify urban areas in the
nine Bay Area counties for 1954 and 1962. Several cities, including
Sacramento, were not included in the ABAG maps so supplemental data were
obtained from USGS 1:24,000-scale topographic maps. The 1990 data layer, based
on the TM imagery, was used as a base to identify ABAG urban polygons that
would be included and those that would be excluded. Urban areas that were on
the 1990 data layer (i.e. that could be identified in the satellite imagery)
were included in the 1954 and 1962 data layers, if they existed. Those urban
polygons that were in the 1954 and 1962 data layers but did not exist in the
1990 coverage were deleted. For example, areas classified as institutional
and military were found to include large undeveloped areas located outside the
closest urban boundary. These classes were either reinterpreted and edited to
include only the developed portions of those categories, or deleted.

1900 and 1940. Historical topographic maps were used to identify urban
areas for 1900 and 1940. The 1900 urban extent was based on topographic maps at
a scale of 1:62,500 ranging in dates from 1897 to 1906. The Sacramento area
was mapped from a 1:125,000-scale map published in 1887 and from a
1:250,000-scale map representative of the Sacramento Valley from 1903 to 1910.
The 1900 maps did not contain the characteristic urban tint, so street patterns
and street density were used to define the urban boundary. USGS and Army
Service maps derived from aerial photos taken 1937 to 1940 were used to create
the 1940 data layer.

The topographic maps typically contained only one or two urban areas per map,
so a large quantity of maps were required to cover the entire study area. We
were unable to delineate urban areas directly on to the topographic maps, so
mylar was overlayed on the maps and the urban areas with associated tic
coordinates were transferred. The mylar overlays were then digitized and input
into a GIS system. After each map was digitized, the polygons were labeled
with their city names. Finally, all polygons for the year being assembled were
integrated to create a complete data layer.

In order to ensure that the data for each year represented all known urban
areas, numerous iterations of gathering maps, transferring information to
mylar, digitizing and displaying resultant polygons was required. We used GIS
capabilities to overlay the polygons for each year on each other to check for
inconsistencies. When the data layer for a particular year was completed, it
was converted into raster form with a 30 meter grid cell size.

Transportation

In addition to the urban extent, historical transportation data layers were
also assembled in the GIS. The transportation layers were created for the same
years as those for the urban extent: 1850, 1900, 1940, 1954, 1962, 1974 and
1990. The 1990 data layer was derived from USGS 1:100,000-scale
Digital Line Graphs (DLGs) of the region. We were only interested in the primary road
network , so only Class 1 and 2 roads were included. We defined the primary
road network as those roads that have been numbered by the Federal
transportation department (US DOT) or the State Department of Transportation
(Caltrans). As a result, many Class 2 roads were also deleted. DLGs were
derived from data collected between 1976 and 1982.To update the 1990
data layer, the DLG was overlaid on to the 1990 Landsat TM scene and any
changes (primarily additions) were made by digitizing on-screen.

Historic transportation maps were obtained from the California Department of
Transportation (Caltrans). We identified those roads with numbers as primary
roads and used the 1990 data layer to create the historical data layers.
Several roads that were primary in earlier years, i.e. 1954, were not
considered primary roads in 1990. The original DLG was used to identify those
roads and add them to the data layer. The result was one data set that
included all primary roads for all six years. The attributes assigned to each
road included whether it was primary for a certain year (Y or N) and what the
number of the road was for each year. The GIS enabled us to select only those
roads that were primary for each respective year.

Visualization

The temporal GIS built for the study area ideally lends itself to time-series
animation and data visualization. The seven data layers of urban extent were
converted into raster images and used to produce and single frame animation or
movie depicting the growth patterns experienced over time.

The San Francisco-Sacramento animation (140 years) consisted of the seven
original source data layers and an additional 133 images created by linear
interpolation. The additional images were necessary to produce a smooth
animation. Since video resolution is rather coarse, the original vector data
was gridded to a 300-meter spatial resolution. The image interpolation uses a
set of primary images (i.e. 1900 and 1940) as input using the first as a
starting date and the second as the ending date. The program written for the
study computes a linear distance to the ending boundary for each pixel in the
area between the starting and ending boundaries and outputs images based on the
pixels distance.

The yearly frames were then constructed by compositing the desired data
layers. For San Francisco, we first overlayed built-up areas on top of a
shaded relief image base. We then overlayed water bodies and a shoreline on
top of the built-up and shaded relief composite. Other overlays included text
indicating the year and title.

A video was created using specialized video recording equipment. The
equipment allowed us to record single images on a computer screen to the frames
on a video tape. For San Francisco, we recorded each of our images to six
frames on the video tape, recording a total of 840 frames. An additional 30
frames of the 1850 and 1990 images were recorded to start and end the movie,
for a total of 900 frames on the video tape. Since video tape is played back
at 30 frames per second, the San Francisco movie runs 30 seconds.

RESULTS

Figure 3 illustrates the data layers created in the GIS and utilized in
the animation which corresponds with the areal data in Table 2 and the
population data in Table 3. These data layers exemplify the explosive growth
that has occurred in the San Francisco-Sacramento urbanized region over time.
The pattern of development in the San Francisco area has been limited by
physical constraints of the bay and the surrounding steep hillsides.
Development in this area first centered around the ports of San Francisco and
Oakland with a smaller

Table 2. Growth of San Francisco-Sacramento urbanized region over time.

commercial area centering around San Jose in the south bay. Sacramento's
development began at the junction of the Sacramento and American Rivers and
eventually extended into their floodplains and up into the foothills of the
Sierra Nevada mountains.

As roads and other forms of transportation became more developed, the
migration of middle-income residents from the central city to the suburbs
steadily increased between 1900 and 1940. As developable land became
increasingly scarce, the bay was infilled in some areas to allow for
additional development. The largest wave of suburban expansion occurred between
1940 and 1954 as federal and state housing and highway construction programs
proliferated to meet the needs of service personnel returning from World War II
(Gerkens, 1988). This growth continued well in to the 70s. Although growth in
the San Francisco region slowed considerably between 1974 and 1990, the
Sacramento Valley region continues to grow at an alarming rate. The interstate
highway that connects the San Francisco and Sacramento regions has now, itself
become an urbanized corridor, transforming once two separate urban areas into
one large urbanized region. Although the travel distance between the cities of
San Francisco and Sacramento is approximately two hours, the gap between their
adjacent suburban areas has disappeared.

CONCLUSION

The USGS has traditionally utilized various methods to map land use/land
cover, but this project signifies the first attempt to focus on urban regions.
The methodology developed here integrates historical maps, topographic maps,
digital imagery and animation techniques to portray a unique perspective of
regional urban development over time. We intend to broaden the scope of this
mapping project by incorporating population density, wetlands, and protected
lands. Historical topographic maps and satellite imagery is available for
every large metropolitan area in this country. Producing dynamic maps of these
areas will enable scientists, planners, policy makers and teachers to better
understand the impacts of growth on the regional and global environments and to
model future growth.

REFERENCES

Anderson, J.R., E.E. Hardy, J.T. Roach, and R.E. Witmer, 1976. "A Land Use and
Land Cover Classification System for Use with Remote Sensor Data", US
Geological Survey, Professional Paper 964.

Gerkens, L.C., 1988., "Historical Development of American City Planning" in
Frank So et. al. (eds.), The Practice of Local Government Planning, Washington
DC: International City Management Association, pp. 32.

Jensen, John R., 1983. "Urban/Suburban Land Use Analysis" in Manual of Remote
Sensing, Vol. II, Edited by John Estes, Falls Church, Virginia, American
Society of Photogrammetry pp. 1571-1666.